Search PubMed⌕ Search

Biomedical subjects

F Clarac

Publications and source records attributed to F Clarac.

At least 37 records · Page 2Linked to original sources
PubMed1998

Role of gravity in the development of posture and locomotion in the neonatal rat.

This report describes the early motor behaviour in the neonatal rat in relation with the maturation of sensory and motor elements of the central nervous system (CNS). The role of vestibular information during the week before (E14-21) and the 2 weeks after (P0-15) birth will be considered. There is a rostro-caudal gradient in the maturation of posture and locomotion with a control of the head and forelimbs during the first postnatal week and then a sudden acceleration in the functional maturation of the hindlimb. At birth, the neonatal rat is blinded and deaf; despite the immaturity of the other sensory systems, the animal uses its olfactory system to find the mother nipple. Vestibular development takes place between E8 and P15. Most descending pathways from the brainstem start to reach the lumbar enlargement of the spinal cord a few days before birth (reticulo-, vestibulospinal pathways as well as the serotonergic and noradrenergic projections); their development is not completed until the end of the second postnatal week. At birth, in an in vitro preparation, a locomotor activity can be evoked by perfusing excitatory amino acids and serotonin over the lumbar region. The descending pathways which trigger the activity of the CPG are also partly functional. At the same age both air stepping and swimming can be induced. Complex locomotion such as walking, trotting and galloping start later because it requires the maturation of the vestibular system, descending pathways and postural reflex regulation. The period around birth is critical to properly define how the vestibular information is essential for the structuring of the motor behaviour. Different types of experiments (hypergravity, microgravity) are planned to test this hypothesis.

Aging↗
PubMed1998

Early walking in the neonatal rat: a kinematic study.

The development of the early stage of locomotion (between Postnatal Days 3 and 10) was studied in newborn rats. At this age, rats are known to perform limited locomotor activities, consisting of an inefficient nonpostural gait termed crawling. By providing appropriate olfactory stimulation, it was possible to override the pups' reluctance to walk and to discover their actual locomotor abilities. The step period decreased from 1,200 ms to 900 ms from Postnatal Days 4 to 9, showing both a regular decrease in the swing and a discontinuous decrease in the stance phase. The fore- and hindlimb periods stabilized early on an alternate pattern of coupling. The ipsilateral coupling shifted progressively from 220 degrees to 260 degrees in relation with the change in the gait pattern. In parallel with the change in timing, the newborn rats showed gradual changes in the foot position and in the interlimb spatial coordination. These results show that quadruped locomotion develops before postural control is acquired, in a continuous process as the nervous system develops.

Animals↗
PubMed1998

Curve walking in freely moving crayfish (Procambarus clarkii)

The curve walking of freely moving crayfish trained to walk along a curved path during homing behaviour was investigated using a video-analysis system. The leg kinematics and leg phase relationships, as well as the relationship between stepping patterns and body axis rotation measured relative to external references, were studied. The anterior and posterior extreme positions of the power stroke (AEP and PEP, respectively) and step amplitudes were analysed. As in a previous study on crayfish curve walking on a treadmill, PEPs were more posterior in outer legs (the legs on the outside of the turn) than in the inner legs. As a result, outer legs showed larger step amplitudes than inner legs. Leg kinematics varied within each walking sequence. AEP leg angles (the angles between the body and leg axes at the AEP) tended to decrease over time for inner legs and increase for outer legs. This leg angle drift was present mainly in the anterior legs and it suggests that these legs did not completely compensate for the body rotation after each step. In addition, leg angle asymmetries in a direction opposite to that of leg angle drift were observed at the start of each curve-walking sequence, suggesting that the extensive training (3 weeks) may have allowed crayfish to anticipate the leg angle drift. The rotational component of curve walking showed a discontinuous pattern, with the animal's body axis turning towards the inside of the curve only periodically. Analysis of cross-correlation functions showed that the angular acceleration of the body axis in the direction of the turn occurred during the power strokes of inner legs 2 and 5 and outer leg 4. While the tripod formed by these three legs showed in-phase relationships, the legs of the corresponding contralateral tripod (outer legs 2 and 5 and inner leg 4) were not in phase. We hypothesize that inner legs 2 and 5 and outer leg 4 act synergically causing the inward body rotation observed in curve-walking crayfish and that some of the asymmetries found in step geometry may be a passive phenomenon due to the body rotation.

Journal Article↗
PubMed1997

Modulation of the spinal network for locomotion by substance P in the neonatal rat.

The tachykinin substance P (SP) is present in the ventral and medial area of the lumbar spinal cord. Its localisation suggests that it could modulate the spinal network for locomotion. We have investigated its effects on motor outputs by applying SP, in vitro, to the lumbosacral segments of an isolated spinal cord of new-born rats. SP was applied to the lumbosacral spinal cord either on a quiescent preparation or during episodes of fictive locomotion induced by N-methyl-D,L-aspartate. When applied on quiescent preparations, SP induced a slow rhythmic activity (period >30 s). During fictive locomotion, SP increased both the locomotor frequency and the duration of the bursts of cyclic activity. Furthermore, SP stabilised the locomotor rhythm. These results demonstrate that SP is able to modulate both the "clock" and the pattern generator for locomotion.

Animals↗
PubMed1997

Antidromic modulation of a proprioceptor sensory discharge in crayfish.

In the proprioceptive neurons of the coxo-basal chortotonal organ, orthodromic spikes convey the sensory information from the cell somata (located peripherally) to the central output terminals. During fictive locomotion, presynaptic depolarizations of these central terminals elicit bursts of antidromic spikes that travel back to the periphery. To determine whether the antidromic spikes modified the orthodromic activity of the sensory neurons, single identified primary afferents of the proprioceptor were recorded intracellularly and stimulated in in vitro preparations of crayfish nervous system. Depolarizing current pulses were delivered in trains whose frequency and duration were controlled to reproduce bursts of antidromic spikes similar to those elicited during fictive locomotion. According to their frequencies, these antidromic bursts reduce or suppress the orthodromic discharges in both position- and movement-sensitive neurons. They induce both a long-lasting silence and a gradual recovery after their occurrences. Neither the collision between the afferent and the efferent messages nor the release of serotonin by the sensory neurons can explain these results. We therefore conclude that antidromic bursts produce a peripheral modulation of the orthodromic activity of the sensory neurons, modifying their sensitivity by mechanisms yet unknown.

Animals↗
PubMed1997

Functional analysis of the sensory motor pathway of resistance reflex in crayfish. I. Multisensory coding and motor neuron monosynaptic responses.

An in vitro preparation of the fifth thoracic ganglion of the crayfish was used to study in detail the negative feedback loop involved in the control of passive movements of the leg. Release-sensitive primary afferents of from the coxo-basipodite chordotonal organ (CBCO), a proprioceptor whose strand is released by upward movement of the leg, monosynaptically connect to depressor motor neurons (Dep MNs). Extracellular identification of sensory units from the CBCO neurogram allowed us to determine the global coding of a sine-wave movement, imposed from the most released position of the CBCO strand. Intracellular recordings from sensory terminals (CBTs) and ramp movement stimulations applied to the CBCO strand allowed us to characterize two groups of release-sensitive CBCO fibers. The first group, divided into two subgroups (phasic and phaso-tonic), is characterized by discontinuous firing patterns: phasic CBTs fired exclusively during release movements; phaso-tonic CBTs displayed both a phasic firing and a tonic discharge during the more released plateaus. The second group was continuously firing whatever the movement, with higher frequencies during the release phase of the movement stimulation. All CBTs displayed a marked sensitivity for release movements while only the phaso-tonic ones showed a clear sensitivity to maintained positions. Surprisingly, no pure tonic sensory fibers were encountered. Systematic intracellular recordings from all resistant Dep MNs, performed in high divalent cation saline, allowed us to describe two shapes of monosynaptic resistance reflex responses. A phasic response was characterized by bursts of excitatory postsynaptic potentials (EPSPs) occurring exclusively during CBCO strand release movements. A phaso-tonic response was characterized by a progressive depolarization occurring all along the release phase of the stimulation: during maintained released positions, the amplitude of the sustained depolarization was position dependent; in addition, each release movement produced a phasic burst of EPSPs in the MN. The parallel study of the Dep MN properties failed to point out any correlation between the type of reflex response recorded from the MN and the MN intrinsic properties, which would indicate that the type of MN response is entirely determined by the afferent messages it receives.

Animals↗
PubMed1997

Functional analysis of the sensory motor pathway of resistance reflex in crayfish. II. Integration Of sensory inputs in motor neurons.

The in vitro preparation of the fifth thoracic ganglion of the crayfish was used to analyze the connections supporting the monosynaptic reflex responses recorded from the depressor motor neurons (Dep MNs). Dep MNs are directly connected by the release-sensitive afferents from a proprioceptor, the coxo-basipodite chordotonal organ (CBCO), which is released by upward movements of the leg. Sine-wave movements, applied to the CBCO strand from the most released position, allowed us to stimulate the greatest part of release-sensitive CBCO fibers. Systematic intracellular recordings from all Dep MNs performed in high divalent cation saline allowed us to determine the connections between CBCO afferents and their postsynaptic Dep MNs: it highlighted the sequential activation of the different Dep MNs involved in the monosynaptic reflex. The convergence of different sensory afferents onto a given Dep MN, and the divergence of a given sensory afferent onto several Dep MNs illustrates the complexity of the sensory-motor reflex loops involved in the control of locomotion and posture. Electrophysiological experiments and simulations were performed to analyze the mechanisms by which Dep MNs integrate the large amount of sensory input that they receive. Paired intracellular recording experiments demonstrated that postsynaptic response shapes characteristic of both phasic and phaso-tonic afferents could be induced by varying the presynaptic firing frequency, whatever the postsynaptic Dep MN. Compartment model simulations were used to analyze the role of the sensory-motor synapse characteristics in the summation properties of postsynaptic MN. They demonstrated the importance of the postsynaptic compartment geometry, because large postsynaptic compartments allowed to generate greater excitatory postsynaptic potential (EPSP) summations than small ones. The results presented show that velocity information is the most effective to elicit large compound EPSPs in MNs. We therefore suggest that the negative feedback reflex is mainly based on the detection of leg movements.

Animals↗
PubMed1997

Variability of leg kinematics in free-walking crayfish, Procambarus clarkii, and related inter-joint coordination

The inter-joint coordination in leg 4 of the crayfish Procambarus clarkii was investigated while they travelled freely along straight paths. Video analysis of the kinematics of the leg's three-dimensional motion, combined with a statistical method based on conjugate cross-correlation functions, showed stable inter-joint coordination in the leg kinematics. In particular, the inter-joint coordination involved a strong movement in the distal mero-carpopodite (M-C) joint occurring between the movements of the proximal thoraco-coxopodite and coxo-basal joints; thus, the leg extended during the swing phase and flexed during the stance phase. This synchronisation was mainly independent of global changes in the locomotor pattern induced by variation in speed or contralateral inter-leg coupling which occurred during free walking. The main changes in inter-joint coordination were found to be related to the appearance of a biphasic flexion/extension movement during each stroke of the step cycle when the leg retracted far backwards. This movement was observed more frequently in large animals and was therefore possibly related to changes in postural control. The functional role of this distal M-C joint movement in the leg motion is discussed.

Journal Article↗
PubMed1996

CGP 35348 and CGP 55845A block the baclofen-induced depression of dorsal root evoked potentials in lumbar motoneurons of the neonatal rat.

In vitro brainstem-spinal cord preparations isolated from neonatal (0-5 days old) rats were used to investigate the GABAB receptor-mediated modulation of the dorsal root evoked potentials in lumbar motoneurons recorded intracellularly. The GABAB receptor agonist, baclofen, at low concentrations (1-10 microM), caused a reduction of the amplitude of the monosynaptic excitatory postsynaptic potential (EPSP), in a concentration-dependent manner. The depression of EPSPs was likely exerted at a presynaptic level since it occurred without any significant change of the passive membrane properties of the motoneurons. The two GABAB receptor antagonists, CGP 35348 and CGP 55845A blocked the effects of baclofen. These two compounds may be useful tools to study the evolution of GABAB receptor-mediated presynaptic inhibition during ontogenesis.

Animals↗
PubMed1996

Synaptic connections of the cuticular stress detectors in crayfish: mono- and polysynaptic reflexes and the entrainment of fictive locomotion in an in vitro preparation.

The reflex connections made by Cuticular Stress Detector afferents (CSD1 and CSD2) with motorneurones of the four proximal muscle groups in the 5th walking legs of crayfish (Procambarus clarkii, Pacifastacus leniusculus) have been studied in an in vitro preparation. Reflex responses to mechanical stimulation of the CSDs were studied in single neurones by means of intracellular techniques. Within each motorneurone pool, both excitatory and inhibitory reflex responses occurred, although sometimes no reflex connections were found. When present, they could be classified into 'levation' and 'depression' reflexes, corresponding to negative and positive feedback effects respectively. Each motorneurone receives input from a number of different CSD afferents (mean values between 3.0 and 5.8). Using electrophysiological and pharmacological tests, it was demonstrated that at least 32% of all connections were monosynaptic. In preparations showing fictive locomotion, phasic CSD stimulation was shown to be able to entrain anterior levator and depressor motorneurone activity in 95% of cases. The results thus demonstrate the importance of sensory feedback from the CSDs in shaping the final motor output.

Animals↗
PubMed1996

The synaptic drive from the spinal locomotor network to motoneurons in the newborn rat.

The nature of the synaptic drive from the locomotor spinal network onto the motoneurons was studied in the newborn rat. For this purpose, an in vitro isolated spinal cord preparation of newborn rat was used. The recording chamber was partitioned with Vaseline walls to separate the L1/L2 lumbar segments, in which the spinal locomotor network is located, from the motoneurons in the lower lumbar segments. Locomoter-like activity was induced by bath-applying a mixture of serotonin and NMDA to segments L1/L2. In this way, the synaptic activity could be modified at the lower lumbar level without affecting the motor pattern. The drive elicited onto the motoneurons during sequences of locomoter-like activity, which was monitored by performing intracellular recordings, consisting of an inhibitory component followed by an excitatory component. The inhibitory synaptic volley was reversed at a membrane potential of --60 mV with K acetate electrodes, whereas it was shifted toward positive values with KCl electrodes. The glycinergic blocker strychnine, bath-applied to segments L3/L5, blocked the inhibitory drive without affecting the rhythmic activity, whereas it disrupted the locomoter-like activity when bath-applied to segments L1/L2. The inhibitory part of the drive was more sensitive than the excitatory part to changes in the membrane potential. The excitatory phase was mixed and consisted of an NMDA and a non-NMDA component, which were sensitive to 2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, respectively. It was concluded that the locomotor network located in segments L1/L2 sends a biphasic projection to the various groups of motoneurons located along the lumbar spinal cord.

Animals↗
PubMed1995

Evidence for the existence of a functional polysynaptic pathway from trigeminal afferents to lumbar motoneurons in the neonatal rat.

Stimulation of trigeminal afferents has been reported to have powerful effects on the spinal cord in adult animals of several species. In the present study, the pathway transmitting these influences was investigated in the neonatal rat. Experiments were performed on in vitro brainstem/spinal cord preparations. Stimulation of the trigeminal nerve evoked bilateral polysynaptic discharges in lumbar ventral roots. Intracellular recordings from lumbar motoneurons showed mainly excitatory responses, although a few inhibitory responses were also observed. Experiments with perfusion of different parts of the preparation with general or selective synaptic blockers revealed a synaptic relay under GABAergic control in the brainstem, and at least one synapse in the cervical and in the thoracic spinal cord. The involvement of lumbar interneurons was established by perfusing the lumbar enlargement with saline containing either a high concentration of divalent ions or mephenesin in order to reduce transmission along polysynaptic pathways. The contribution of excitatory amino acid transmission was evaluated and was found to evoke mixed receptor responses. The course of the pathway was traced by using different lesions to the brainstem and spinal cord. The pathway was found to be ipsilateral in the brainstem and to become bilateral in the spinal cord. The results of the present study demonstrate that polysynaptic sensorimotor pathways are present at birth. The results are discussed in relation to the pontomedullary locomotor strip, which has been thought to share many features with the trigeminal system.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1995

Cholinergic control of the walking network in the crayfish Procambarus clarkii.

The output of a neuronal network results generally from both the properties of the component neurons and their synaptic relationships. This article aims at synthesizing various results obtained on the neural network generating locomotion in vitro. In the preparation used, consisting of the last three thoracic ganglia (3-5) along with motor nerves from the 5th leg ganglion to the promotor, remotor, levator and depressor muscles, motor nerve recordings generally revealed only tonic activity in several different motoneurons (MNs). However, rhythmic activity can be obtained by the use of cholinergic agents such as the oxotremorine (Oxo) superfused in the bath (5 x 10(-5) M). If Oxo is pressure-ejected locally in the ganglion, it is possible, depending upon the locus where the drug is applied, to elicit a rhythmic activity restricted to a group of antagonistic MNs. To analyze how cholinergic agents are able to induce such rhythmic activity, very small volumes of drug (50-200 pl), were applied close to the recording electrode. Two types of depolarizing response occurred: a fast large amplitude depolarization (5-20 mV) and a long lasting (10s to several minutes) low amplitude depolarization (1-3 mV). These responses persisted in the presence of TTX and Co(2)+. The transient initial depolarization is a mixed nicotinic and muscarinic voltage-independent response during which the input resistance decreases by 20 to 40%. In contrast, the long lasting component is voltage-dependent, exclusively muscarinic and associated to a 5-10% increase of input resistance due to the closing of a K+ conductance that is active at the resting Vm, and totally suppressed at holding potentials below -70 mV. More generally, K+ currents activated at resting potential are responsible for membrane potential stability. The injection of TEA, a blocker of the K+ currents, through the recording electrode is able to unmask plateaus above a threshold depolarization. These plateaus are TTX-sensitive but persist in the presence of Ca(2)+ channel blockers. Moreover, in 10% of TEA-filled MNs a spontaneous pacemaker activity was revealed. The organization of the locomotor network is also based upon connections between MNs and INs. Within a MN pool, connections are only loosely established, appearing to consist mainly of electrical coupling. Inhibitory synaptic connections between MNs of opposite pools are mediated by chloride channels. However, the neurotransmitter involved could be either GABA or glutamate. Therefore, at the level of a given joint, a basic rhythm occurs due to both motoneuronal membrane properties and motoneuronal connectivity. However, the coordination of all MNs of an entire leg during fictive walking activity requires the involvement of INs. Based upon these data, we propose a two-stage model of the locomotor network organization: a joint motoneuronal level and a whole leg interneuronal level.

Acetylcholine↗
PubMed1995

Locomotor patterns in freely moving crayfish (Procambarus clarkii)

Freely walking crayfish, Procambarus clarkii, were studied using a video analysis procedure adapted especially for use with crayfish. The animals were placed in a tank and their homing behaviour was filmed as they returned in a straight line to their shelter. Various sequences were studied at the two following levels. First, the trajectory of each pair of legs (from leg 2 to leg 5) during the step cycle (power stroke and return stroke) was studied to measure stride length and to analyse in detail changes in acceleration. Each leg was found to contribute in a specific manner to locomotion. Second, ipsi- and contralateral leg coordination was investigated. Ipsilateral coordination was found to involve a metachronal organization from front to back in all the walking sequences recorded, whereas contralateral coordination involved, in addition to the weak alternate coupling commonly observed in treadmill walking, another coordination pattern where the legs on each side (legs 3 and 4) are in phase. The results obtained in these free-walking sequences are discussed and compared with those obtained previously, in particular in treadmill situations.

Journal Article↗
PubMed1995

Localization and organization of the central pattern generator for hindlimb locomotion in newborn rat.

An in vitro preparation of newborn rat isolated brainstem/spinal cord was used in order to locate the spinal network responsible in mammals for producing patterned locomotor activity. The spinal cord was partitioned by building Vaseline walls at various lumbar levels. When a mixture of serotonin and N-methyl-D,L-aspartate was bath applied to the upper lumbar cord (L1/L2 segments), rhythmic locomotor-like activity was induced and recorded in all the lumbar segments (from L1 to L5). Conversely, when the mixture of transmitters was bath applied to the lower lumbar cord, only tonic activity was induced in the lower lumbar segments. Intracellular recordings performed on motoneurons revealed that during elicited L1/L2 locomotor-like activity, they received a rhythmic synaptic drive that was often below the threshold for spiking, because the excitability of the neurons was too low. When the L1/L2 segments were isolated, their burst production capacities remained. The network located at the L1/L2 level was found to be responsible not only for generating the rhythm but also for organizing its alternating pattern. We demonstrated that the rhythmic synaptic drive that the motoneurons receive during locomotor-like activity comes directly from the L1/L2 network and that there is no relay at the segmental level. We conclude from our study that the network that organizes locomotion in the newborn rat is not segmentally distributed but is restricted to a specific part of the cord. This finding has important consequences, since it means that it is now feasible to study the activity of the rhythmic spinal network independently from that of the motoneurons.

Animals↗
PubMed1994

Chloride conductance produces both presynaptic inhibition and antidromic spikes in primary afferents.

Primary afferents from a crayfish leg proprioceptor display both primary afferent depolarizations (PADs) and antidromic spikes. PADs are generated by activation of GABA receptors and produce presynaptic inhibition, while the antidromic spikes do not elicit any synaptic effect in the postsynaptic neurons. The aim of the present study was to investigate the ionic mechanisms that allow PADs to produce antidromic spikes and to test whether GABA can produce similar effects. Intracellular recordings from the sensory axon terminals within the ganglion where PAD are produced were performed. Lowering the extracellular chloride concentration resulted in an increase in PAD amplitude, which was then capable of producing antidromic spikes. Local application of GABA close to the axon terminal also resulted in production of antidromic spikes. We conclude that antidromic spikes may result from the activation of a GABA-mediated increase in chloride conductance that also produces PADs. Therefore PADs and antidromic spikes may represent two aspects of the same GABAergic inhibitory mechanism that gate sensory transmission.

Action Potentials↗
PubMed1994

Inhibitory effects of L-glutamate on central processes of crustacean leg motoneurons.

In crustaceans, glutamatergic excitation at the neuromuscular synapse has been extensively studied. Fewer reports exist of the central and possibly inhibitory actions of glutamate on neurons. The present study analyses the response of intracellularly identified motoneurons, which innervate the proximal leg muscles, to local glutamate pressure applications in the neuropil, in an in vitro thoracic preparation of the crayfish Procambarus clarkii. L-Glutamate application always inhibited motoneuron activity, with a decrease in input resistance. The resulting depolarization or hyperpolarization could usually be reversed within 10 mV of the resting potential. The response persisted in neurons pharmacologically isolated with Cd2+ or tetrodotoxin. The reversal potential of the response to glutamate was displaced in a low-chloride solution. Similar responses were obtained with GABA. Application of GABA blocked the glutamate response in a competitive manner. Both responses were suppressed by beta-guanidino-propionic acid, a competitive antagonist for GABA receptors. This indicates that glutamate activates a chloride-GABA receptor-channel. Micromolar concentrations of picrotoxin reduced both the L-glutamate and the GABA inhibitory responses, thereby unmasking a smaller, picrotoxin-resistant effect of glutamate (but not of GABA), which was excitatory and sensitive to 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results suggest dual and opposite roles for motoneuron glutamatergic connections--a peripheral (well known) net excitatory one and a central net inhibitory one. Direct inhibition of motoneurons by L-glutamatergic neurons is to be expected.

Animals↗